1. Field of the Invention
This invention relates to a radiation dosimeter for measuring radiation dose a place where radiation exposure is likely to occur, such as in space or at a nuclear reactor site and, more particularly, to an apparatus that measures radiation dose using a semiconductor device and can be reconditioned for repeated use by electrically removing electric charge building up therein owing to exposure to radiation.
2. Description of the Related Art
When an MOS semiconductor device is used under exposure to radiation, such as in space or at a nuclear reactor site, its operation becomes defective when the total radiation dose received reaches around 1.times.10.sup.6 RAD.
FIGS. 19 and 20 are graphs for explaining why the operation of the MOS semiconductor device becomes defective under exposure to radiation.
The characteristic curve of FIG. 19 shows how the threshold voltage of an N-channel MOS semiconductor device changes with radiation dose. The horizontal axis represents radiation (gamma-ray) dose and the vertical axis the amount of change in threshold voltage (.DELTA.Vth) caused by the radiation.
As shown in FIG. 19, the threshold voltage of the N-channel MOS semiconductor device decreases with increasing dose. The leakage current of the N-channel MOS semiconductor device therefore increases.
The characteristic curve of FIG. 20 shows how the threshold voltage of a P-channel MOS semiconductor device changes with radiation dose. The horizontal axis represents radiation (gamma-ray) dose and the vertical axis the amount of change in threshold voltage (.DELTA.Vth) caused by the radiation.
As shown in FIG. 20, the threshold voltage (absolute value) of the P-channel MOS semiconductor device increases with increasing dose. The P-channel MOS semiconductor device therefore turns off and becomes inoperative.
An MOS semiconductor device of either the N-channel or P-channel type used under exposure to radiation thus experiences a progressive change in threshold voltage with increasing radiation dose.
Since the threshold voltage shift is in the negative direction, it follows that the charge produced by radiation exposure is positive. The occurrence of this positive charge in the gate oxide film has been observed.
A system incorporating MOS semiconductor devices and used under exposure to radiation will therefore become inoperative as the radiation dose increases.
MOS semiconductor devices made inoperable by radiation exposure are ordinarily reconditioned by annealing at a temperature between 300.degree. C. and 400.degree. C. to remove the positive charge accumulated in the gate oxide film.
Since complete removal of positive charge by annealing is difficult, however, the total radiation dose a system can tolerate is limited. System replacement is therefore necessary after a certain period of use.
Another problem is that the system is undesirably enlarged by the need to incorporate a device for effecting the annealing. As the conventional system provides no information regarding the radiation dose, moreover, accurate prediction of when operation failure will occur is impossible. Since the only alternative is to conduct annealing at regular time intervals, the efficiency of positive charge removal cannot be optimized when the system is used in an environment where the radiation dose varies continually.
The MOS semiconductor device is also used as a dosimeter which measures radiation dose based on the radiation damage it incurs. Although this type of dosimeter has the merits of small size, wide dose measurement range and low cost, its sensitivity is insufficient for use in very low radiation dose regions and its reproducibility poor.
Another dosimeter employed measures radiation dose by using a combination of an operational amplifier and a capacitor as a charge amplifier to integrate ionization current from a silicon diode or a solar cell used as a solid-state ionization chamber.
This dosimeter has the drawback of being enlarged by the size of its complex integrator circuit. Further, since the characteristic of its operational amplifier would be degraded by radiation damage if installed at a high-intensity radiation site, the integrator has to be separately placed in a safe area and supplied with the ionization current through a cable.
This invention was accomplished in response to these circumstances and is directed to providing a compact and simple radiation dosimeter which utilizes the aforesaid change in the threshold voltage of a semiconductor device with radiation exposure to accurately measure radiation dose at a place exposed to radiation, such as in space or at a nuclear reactor site, and which can be reconditioned for repeated use by electrically removing electric charge building up therein owing to exposure to radiation.